There is an increasing need to develop strategies for pulp regeneration therapy to overcome tooth injury due to caries, restorative procedures, or trauma. Currently, there is a significant gap in our understanding of the molecular and cellular mechanisms regulating reparative dentinogenesis. Our long-term goal is to gain fundamental knowledge on the human dental pulp cellular niche and to apply that knowledge to lessen the burdens of tooth injury due to caries, restorative procedures, or trauma. The objective for this application is to elucidate how the interactions between ephrinB1 and IGF-1 in the dental pulp niche induce dentinogenesis in vivo. The overarching hypothesis is that IGF-1 regulates cells of the tooth pulp niche, and induces dentinogenesis via ephrinB1. Guided by strong preliminary data, this hypothesis will be tested by pursuing the following two specific aims: Aim 1: Determine the mechanism by which ephrinB1 controls the number of odontoblast progenitors, their proliferation, and differentiation in the tooth pulp. And Aim 2: Determine the cellular and molecular mechanisms by which IGF-1 regulates ephrinB1 expression in vivo using mouse models and ex vivo using human DPSCs and human oral mucosa stem cells. An already-generated odontoblast-specific ephrinB1 knockout mouse lines (using cre driven by the DMP1 or osteocalcin promoters), and mouse lines of IGF-1 receptor (DMP1-IGF-1RKO) and the hepatic IGF-1 transgenic (HIT) line will be used to achieve the two aims. Importantly, initial characterization of our models indicates that both IGF-1 and ephrinB1 involved in dentinogenesis in vivo. The proposed research is conceptually innovative because we show, for the first time, the interactions between ephrinB1 and IGF-1 during dentinogenesis in vivo. Further, we offer a direct approach to determine these interactions using unique mouse models, as well as primary human dental pulp cell cultures. The proposed research is significant because it is expected to advance the field of regenerative endodontic procedures and will impact the overall effort to retain the natural tertiary dentin formation process following injury.